Scripps Pier water temperature-taking, Aug. 2018. Photo: Erik Jepsen, UC San Diego

Cause of Historic Ocean Heat Wave off San Diego in 2018

Stifled ocean activity the dominant force behind historic event

Climate researchers at Scripps Institution of Oceanography at UC San Diego identified a new main reason why water temperatures off San Diego reached their highest point in at least a century in 2018.

Previously physical oceanographers at Scripps had pointed to an unusual absence of winds and clouds off the coast as the reason for surface waters warming under the hot summer sun and reaching a temperature of 26.4℃ (79.5℉) at the heat wave’s peak. Three times during August 2018, a new record high water temperature was established, shattering a record that had stood for nearly 90 years, according to data that Scripps Oceanography has collected since 1916.

Research led by Xinyue Wei and Kaiyuan Li, who served as interns with Shang-Ping Xie, a climate scientist at Scripps Oceanography, during a 2019 international undergraduate research program, put a new spin on the event. They found that weather events thousands of miles away from California were the dominant force that essentially stilled local waters enough to let them warm up over a period of several weeks.

Kaiyuan Li (left) and Xinyue Wei led ocean heat wave research under direction of Scripps Oceanography's Shang-Ping Xie
Kaiyuan Li (left) and Xinyue Wei led ocean heat wave research under direction of Scripps Oceanography's Shang-Ping Xie.

Noticing that the warming lasted almost the entire summer, from July through September, the team divided the periods of ocean temperature deviations into sub-monthly (fewer than 30 days), subseasonal (30-120 days), and longer time scales. They found that the subseasonal warming was due to ocean fluctuations that came from the equatorial Pacific, hugging the coast and deepening the upper warm layer. In the week leading to the record heat wave, remote phenomena including tropical storms several hundred miles to the south of San Diego stifled the flow of deep, colder water to the surface, and thickened the thermocline, the layer of the ocean that is nearest to the surface and warmed by sunlight.

The conclusion from Xie’s group highlights the large-scale environment in which the coastal marine heat wave occurred. Scripps colleagues had earlier identified a lack of cloud cover and curtailed winds as the source of the warm ocean off San Diego in a 2020 study. The two results reflect the different approaches to observing the natural world between seagoing oceanographers more concerned with direct ocean measurements and climate scientists who look for global connections  of climate events.

“Coastal waters off California are usually cold even in summer,” said Xie, who directed the research. “This study shows that they can be as warm as Hawaii when waves from the equator and the relaxed winds conspire to shut off the upwelling that pumps cold water from beneath.”

James Fumo, now a graduate student at the University of Hawaii, led the 2020 study and said its goal was to describe the phenomenon and document that a long-term trend of warming caused by human activities such as fossil fuel burning raises the baseline for extreme heat events. Fumo and colleagues concluded that with so much heat energy added to the oceans in general, any warming episode has a substantially improved chance of setting records. 

Studies of the 2018 marine heat wave off San Diego used data collected by Spray gliders. Photo:Robert Todd
Studies of the 2018 marine heat wave off San Diego used data collected by Spray gliders developed at Scripps Oceanography. Photo: Robert Todd

“The 2018 marine heat wave brought unprecedented warming to the Southern California/ Northern Baja region,” Fumo said. “We are glad that our efforts to describe this event and it’s links to climate change have been built upon by Xie. This new study greatly improves our understanding of the drivers of this marine heat wave.”

Xie said he agrees with that premise and that his work illustrates how events in one region influence events in other regions even over great distances. 

About Scripps Oceanography

Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.

About UC San Diego

At the University of California San Diego, we embrace a culture of exploration and experimentation. Established in 1960, UC San Diego has been shaped by exceptional scholars who aren’t afraid to look deeper, challenge expectations and redefine conventional wisdom. As one of the top 15 research universities in the world, we are driving innovation and change to advance society, propel economic growth and make our world a better place. Learn more at www.ucsd.edu.

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